4. THE HUBBLE SPACE TELESCOPE (HST) KEY PROJECT

We briefly summarize below the results from the HST Key Project, and
provide an updated calibration for these data. The primary goals of
the HST Key Project were to discover and measure the distances to
nearby galaxies containing Cepheid variables, calibrate a range of
methods for measuring distances beyond the reach of Cepheids to test
for and minimize sources of systematic uncertainty, and ultimately to
measure Ho to an accuracy of ± 10%. HST provided the
opportunity to measure Cepheid distances a factor of 10 more distant
than could be routinely obtained on the ground. It also presented a
practical advantage in that, for the first time, observations could be
scheduled in a way that optimized the discovery of Cepheids with a
range of periods independent of phase of the moon or weather
(Madore & Freedman
2005).

Cepheid distances to 18 galaxies with distances in the range of 3 to
25 Mpc were measured using WF/PC and (primarily) WFPC2 on
HST. Observations at two wavelengths (V- and I-band) were
made, chosen to allow corrections for dust. The spacing of observations was
optimized to allow for the discovery of Cepheids with periods in the
range of 10 to 50 days. In addition, 13 additional galaxies with
published Cepheid photometry were analyzed for a total of 31 galaxies.

These Cepheid distances were then used to calibrate the Tully-Fisher
relation for spiral galaxies, the peak brightness of Type Ia SNe, the
Dn-
relation for elliptical galaxies, the Surface Brightness
Fluctuation (SBF) method, and Type II supernovae
(Freedman 2001
and references therein). These methods allowed a calibration of distances
spanning the range of about 70 Mpc (for SBF) out to about 400 Mpc for
Type Ia SNe. These results are summarized in
Figure 10. Combining
these results using both Bayesian and frequentist methods yielded a
consistent value of Ho = 72 ± 3 (statistical)
± 7 (systematic) km s-1 Mpc-1.

We update this analysis using the new HST-parallax Galactic
calibration of the Cepheid zero point
(Benedict et al. 2007),
and the new supernova data from
Hicken et al. (2009).
We find a similar value
of Ho, but with reduced systematic uncertainty, of
Ho = 73 ± 2 (random) ± 4 (systematic) km
s-1 Mpc-1. The
reduced systematic uncertainty, discussed further in
Section 4.1 below, results from having a more robust
zero-point calibration based on the Milky Way galaxy with comparable
metallicity to the spiral galaxies in the HST Key Project
sample. Although, the new parallax calibration results in a shorter
distance to the LMC (which is no longer used here as a calibrator),
the difference in Ho is nearly offset by the fact that no
metallicity correction is needed to offset the difference in
metallicity between the LMC and calibrating galaxies.

A primary goal of the HST Key Project was the explicit propagation of
statistical errors, combined with the detailed enumeration of and
accounting for known and potential systematic errors. In
Table 2 we
recall the systematics error budget given in
Freedman et al. (2001).
The purpose of the original tabulation was to clearly identify the
most influential paths to greater accuracy in future efforts to refine
Ho. Here we now discuss what progress has been made,
and what we can expect in the very near future using primarily space-based
facilities, utilizing instruments operating mainly at mid-infrared and
near-infrared wavelengths.

Identified systematic uncertainties in the HST Key Project
determination of the extragalactic distance scale limited its stated
accuracy to ± 10%. The dominant systematics were: (a) the zero
point of the Cepheid PL relation, which was tied directly to the
(independently adopted) distance to the LMC; (b) the differential
metallicity corrections to the PL zero point in going from the
relatively low-metallicity (LMC) calibration to target galaxies of
different (and often larger) metallicities; (c) reddening corrections
that required adopting a wavelength dependence of the extinction curve
that is assumed to be universal; and (d) zero-point drift, offsets,
and transformation uncertainties between various cameras on HST and on
the ground. Table 2 compares these uncertainties
to what is now being achieved with HST parallaxes and new HST SNe Ia
distances (Table 2,
Column 3 "Revisions"), and then what is expected to be realized by
extending to a largely space-based near and mid-infrared Cepheid
calibration using the combined power of HST, Spitzer and
eventually the James Webb Space Telescope (JWST) and GAIA.
(Column 4, "Anticipated").

In 2001 the uncertainty on the zero point of the Leavitt Law was the
largest on the list of known systematic uncertainties. Recall that the
Key Project zero point was tied directly to an LMC true distance
modulus of 18.50 mag. As we have seen in
Section 3.1.4
improvement to the zero point has come from new HST parallax
measurements of Galactic Cepheids, improved distance measurements to
the LMC from near-infrared photometry, and measurement of a maser
distance to NGC 4258. We adopt a current zero-point uncertainty of
3%.

We next turn to the issue of metallicity. As discussed in
Section 3.1.3, in the optical,
metallicity corrections remain
controversial. However, by shifting the calibration from the
low-metallicity Cepheids in the LMC to the more representative and
high-metallicity Milky Way (or alternatively to) the NGC 4258
Cepheids, the character of the metallicity uncertainty has changed
from being a systematic to a random uncertainty. We conservatively
estimate that the systematic component of the uncertainty on the
metallicity calibration should now drop to ± 0.05 mag. Including
the recent results from
Benedict et al. (2007)
and Riess et al.
(2009a,
b),
our estimate for the current total uncertainty on
Ho is ± 5%.

In terms of future improvements, as discussed further in
Section 7, with the Global Astrometric
Interferometer for
Astrophysics (GAIA), and possibly the Space Interferometry Mission
(SIM), the sample of Cepheids with high precision trignometric
parallaxes will be increased, and as more long-period Cepheids enter
the calibration both the slope and the zero point of the
high-metallicity Galactic Leavitt Law will be improved. By
extending both the calibration of the Leavitt Law and its application
to increasingly longer wavelengths the effects of metallicity and the
impact of total line-of-sight reddening, each drop below the
statistical significance threshold. At mid-infrared wavelengths the
extinction is a factor of ~ 20 reduced compared to optical
wavelengths. And line blanketting in the mid and near infrared is
predicted theoretically to be small compared to the blue portion of
the spectrum. Direct tests are now being undertaken to establish
whether this is indeed the case and/or calibrate out any residual
impact (Section 7.3).

In principle, a value of Ho having a well determined
systematic error budget of only 2-3% is within reach over the next
decade. It is the goal of the new Carnegie Hubble Program, described
briefly in Section 7.3, based on a
mid-infrared calibration of the extragalactic distance scale using the
Spitzer satellite, GAIA and JWST.